Surface Roughness and its Influence on Overland Flow

Birmensdorf

2003

S. 13

SonderdruckMonographie

109530

The critical factor for defining the relationship between velocity and resistance of overland flow on natural hill-slopes is primarily depending on the magnitude of the coefficient defining surface roughness. Surface runoff velocity and time of concentration which is strongly correlated to overland flow verlocity are often determined by empirical equations (Strickler, Izzard, Hathaway, Widmoser, Petrascheck, Morgali-Linsley ...). All these parameters have in common that they use an empirical roughness coefficient (k, n, c) for different surface covers. In the last ten years about 125 presentative plots were irrigated by means of a transportable spray irrigation installation for large plots (50 m2 up to 100 m2) by the Austrian Department for Avalanche and Torrent Research. More than 250 rain simulation experiments wre done in combination with additional investigations on vegetation and soil parameters. The data of the experiments were used to calculate surface runoff velocity from the ascending and descending branch of the hydrograph. Reproducibility of this method has been proved by measurements of surface runoff velocity during rain simulation experiments in the field by use of a salt tracer. During these experiments the influence of slope length, slope inclination, rainfall and runoff intensity as well as the influence of vegetation cover on overland flow velocity was analysed. Calculated roughness coefficients show a strong dependency on vegetation and vary in a higher extent than can be expressed by the Strickler-roughness coefficient for different natural surface covers. Even on stands with high surface runoff well structured vegetation cover is able to reduce runoff velocity to a high extent. The results prove the importance of accurate surface roughness coefficients for calculation of overland flow velocity in hydrological modelling.